0 Figure 38. Large volumes NW SE Well of water move rapidly from Mammoth Cave Plateau Pennyroyal Plain Figure 39. Water levels in 2 A sinkholes and swallow holes wells completed in sandstone 4 through a well-developed net- and carbonate-rock aquifers Well B Well A 6 work of solution cavities in the respond differently to recharge. Big Clifty Sandstone Perched water 8 St. Louis and Ste. Genevieve Member of Golconda Sinkhole table Changes in the water level in Water table 10 Limestones to discharge at Formation QQQQQQ well A, completed in sandstone, springs or to the Green River. Spring Swallow hole show that the sandstone is 12 Sinkhole The openings were formed by recharged quickly but drains 14 Sinkhole dissolution of the limestones as slowly. In contrast, changes in 16 water moved along bedding Spring the water level in well B, open QQQQestoneQQQ 18 planes and fractures. Green Girkin Limestone to underlying limestone River Ste Genevieve Lim formations, show that recharge 20 to and discharge from the 406 carbonate-rock aquifer are rapid Well 408 B Warsaw Limestone and due to large solution openings Fort Payne Formation 410 St Louis Limestone Chattanooga Shale in the limestone. 412 414 NOT TO SCALE 416 Modified from Brown, 1966; WATER LEVEL, IN FEET BELOW LAND SURFACE and Quinlan and others, 1983 418 39° 420 Sandstone 85° 83° EXPLANATION 422 Pennyroyal 20 Plain Direction of ground-water movement and carbonate- 10 87° 0 JDJDJDJDJD 1953 1954 1955 1956 1957 rock aquifers K E N T U C K Y IN INCHES Mammoth Cave 89° PRECIPITATION, Modified from Brown, 1966 Plateau Mammoth Cave Area 37° in the Pennyroyal Plain, water from surface streams enters un- which small springs discharge at the escarpments bounding SANDSTONE AND CARBONATE- derground solution cavities through swallow holes and sink- the Mammoth Cave Plateau. ROCK AQUIFERS holes. Water also enters solution cavities in the Ste. Genevieve The presence or absence of solution openings affects and Girkin Limestones through sinkholes that have developed aquifer recharge and discharge and is reflected by the water Aquifers in sandstone and carbonate rocks are most wide- in the Big Clifty Sandstone Member of the Golconda Forma- levels in wells completed in different rock types. The water spread in the eastern half of the Nation, but also extend over tion that caps the Mammoth Cave Plateau. The sinkholes on level in well A (fig. 39A), completed in the Big Clifty Sand- large areas of Texas and smaller areas in Oklahoma, Arkan- the plateau are collapse sinkholes that developed when the stone Member of the Golconda Formation (fig. 38), rises sas, Montana, Wyoming, and South Dakota (fig. 4). These sandstone cap collapsed into caves which formed in the un- quickly in response to seasonal increases in precipitation; after aquifers consist of interbedded sandstone and carbonate rocks; derlying limestone. Many of the solution openings in the Girkin the sudden rise, the water slowly drains from the aquifer and the carbonate rocks are the most productive aquifers, whereas and Ste. Genevieve Limestones are dry because they formed the water level declines slowly. In contrast, the water level in the interbedded sandstones yield less water. The aquifers in when the erosional base level in the area was at a higher alti- well B (fig. 39B), which is open to the St. Louis and Ste. the Mammoth Cave area of Kentucky are examples of sand- tude. Water from the saturated solution openings in the Ste. Genevieve Limestones, rises sharply only in response to heavy stone and carbonate-rock aquifers. The development of solu- Genevieve and St. Louis Limestones discharges to the Green rains. Following the abrupt rise, the water level in this well tion openings and karst topography in the limestones of the River from springs in the river channel and valley walls. Large declines quickly as the solution cavities penetrated by the well Mammoth Cave area are discussed in the preceding section quantities of water move rapidly to the river through the solu- are drained. The large openings allow rapid recharge and of this report; the following section discusses the relations of tion openings. equally rapid discharge during and immediately following the sandstone and carbonate-rock aquifers in that area. Water moves slowly through intergranular pore spaces periods of intense precipitation. The transmissivity (rate at Movement of water through the unconfined and confined and small fractures in the Big Clifty Sandstone Member of the which water moves through an aquifer) of the part of the rock parts of the limestone aquifers that underlie the Pennyroyal Golconda Formation. Discontinuous layers of shale in the un- that contains solution openings is extremely high, whereas that Plain and Mammoth Cave Plateau is summarized in figure 38. derlying Girkin Limestone (fig. 38) impede the downward of the undissolved rock between the solution conduits gener- Where the St. Louis Limestone is exposed at the land surface movement of water and create a perched water table from ally is very low. zones, and the thickness of the flow. The cooling rate is most BASALTIC- AND OTHER rapid when a basaltic lava flow enters water. The rapid cool- VOLCANIC-ROCK AQUIFERS ing results in pillow basalt (fig. 40), in which ball-shaped Basaltic masses of basalt form, with numerous interconnected open Aquifers in basaltic and other volcanic rocks are wide- spaces at the tops and bottoms of the balls. Large springs that spread in Washington, Oregon, Idaho, and Hawaii, and extend discharge thousands of gallons per minute issue from pillow and other over smaller areas in California, Nevada, and Wyoming (fig. basalt in the wall of the Snake River Canyon at Thousand 4). Volcanic rocks have a wide range of chemical, mineralogic, Springs, Idaho. Interflow zones are permeable zones that de- Volcanic–rock structural, and hydraulic properties. The variability of these velop at the tops and bottoms of basalt flows (fig. 41). Frac- properties is due largely to rock type and the way the rock was tures and joints develop in the upper and lower parts of each ejected and deposited. Pyroclastic rocks, such as tuff and ash flow, as the top and bottom of the flow cool while the center aquifers deposits, might be emplaced by flowage of a turbulent mix- of the flow remains fluid and continues to move, and some ture of gas and pyroclastic material, or might form as wind- vesicles that result from escaping gases develop at the top of blown deposits of fine-grained ash. Where they are unaltered, the flow. Few open spaces develop in the center of the flow pyroclastic deposits have porosity and permeability charac- because it cools slowly. Thus, the flow center forms a dense, teristics like those of poorly sorted sediments; where the rock low-permeability zone between two more permeable zones. fragments are very hot as they settle, however, the pyroclas- Thin flows cool more quickly than thick flows, and accordingly tic material might become welded and almost impermeable. the centers of thin flows commonly are broken and vesicular Silicic lavas, such as rhyolite or dacite, tend to be extruded like the tops and bottoms of the flows. as thick, dense flows and have low permeability except where The Snake River Plain regional aquifer system in south- R.L. Whitehead 1980 they are fractured. Basaltic lavas tend to be fluid and form thin ern Idaho and southeastern Oregon (fig. 42) is an example Figure 40. Pillow basalt forms when basaltic lava enters water flows that have a considerable amount of primary pore space of an aquifer system in basaltic rocks. The Snake River Plain and cools quickly. Extensive interconnected pore spaces develop in at the tops and bottoms of the flows. Numerous basalt flows is a large graben-like structure that is filled with basalt of Mi- the flow as the ball-shaped pillows cool. commonly overlap and the flows commonly are separated by ocene and younger age (fig. 43). The basalt consists of a large soil zones or alluvial material that form permeable zones. number of flows, the youngest of which was extruded about Basalts are the most productive aquifers of all volcanic rock 2,000 years ago. The maximum thickness of the basalt, as es- types. timated by using electrical resistivity surveys, is about 5,500 The permeability of basaltic rocks is highly variable and feet. The basalt is bounded at the margins of the plain by si- depends largely on the following factors: the cooling rate of licic volcanic and intrusive rocks that are downwarped toward the basaltic lava flow, the number and character of interflow the plain. 120° EXPLANATION r e Snake River Plain regional 48° bia v Flow top– Colum i vesicular, R aquifer system scoriaceous ●Seattle ● and broken; Spokane substantial W A S H I N G T O N permeability ★Olympia 115° SCALE 1:7,500,000 ake 0 2550 MILES Sn 0 2550 KILOMETERS n R a River iv e e r C umbia c o l ● Portland O Flow center– Yellowstone ★Salem dense, few c National Park i vesicles, most f fractures are i vertical; mini- c a Eugene I D A H O mal permeability ● O R E G O N P ★Boise 43° Sn e ak r Pocatello e ● iv ■ R Flow bottom– Thousand vesicular and Springs broken; substan- tial permeability Base modified from U.S. Geological Survey Modified from Whitehead, 1992 Modified from Whitehead, 1992 digital data, 1:2,000,000, 1972 Figure 41. A typical basalt flow contains zones of varying permeability. Vesicular, broken zones at the top and bottom of the flow are highly permeable, whereas the dense Figure 42.